Rhamnolipids are surface-active glycolipids and metabolic products of particular microorganisms. They have particular surfactant properties, such as strong foam formation, for example, and are of interest for a wide variety of technical applications. Rhamnolipids may be prepared using both wild strains and genetically modified microorganisms. Methods for the preparation by fermentation and work-up of rhamnolipids are described in detail in the prior art, thus, for example, in US20130130319.
Surfactants incorporated in cosmetic formulations (e.g. shampoos, hand cleansers), household cleaners and also dishwashing compositions must be present in liquid form at common processing temperatures in order to ensure processing via pipeline systems and pumps in the producer of such products. The viscosity should be low in this case in order to ensure a simple and reliable conveying. At the same time, the highest possible surfactant concentration is nevertheless desired here to allow preparation of environmentally-friendly formulations having low water content. At the same time, the drop in viscosity on dilution with water must be kept as low as possible since the formulated end products, such as shampoos, hand cleansers and detergents, for example, should have a sufficient viscosity so it can be handled better by the user. No rhamnolipid-containing compositions are known from the literature which correspond to these requirements.
The product forms known from the literature and available on the market are either solids which were isolated via various methods in more or less pure form (e.g. removal of a solvent by drying), pure products which are marketed in low and medium aqueous concentrations (e.g. up to 25% by weight) or are liquid products in which the main component(s) is/are vegetable oil or degradation products of vegetable oil originating from fermentation. These are therefore rhamnolipid solutions in an oil phase, and the corresponding products have the critical disadvantage that they only foam very poorly and are therefore uninteresting for actual surfactant applications.
A highly concentrated product form is also known from DE4237334A1, which may be obtained by a particularly simple method for purifying and concentrating rhamnolipids. In this case, the RL is precipitated from a fermentation broth by acidifying to pH=3 and the corresponding RL solid can be concentrated by centrifugation. In this case, solid suspensions or pastes are obtained having a high rhamnolipid solids content of 30 to 40% by weight and a very high viscosity. Abdel-Mawgoud et al. in Appl Biochem Biotechnol. 2009 May; 157(2):329-45 describe a similar method.
The solid dispersions thus obtained are unsuitable for further processing on a large industrial scale, for example, preparation of formulations for shampoos and household cleaners, since they are not suitable for conveying with typically used pumps owing to their high viscosity and their non-homogeneity.
DE102012221519A1 describes a method for obtaining rhamnolipids with excellent foaming properties, in which aqueous rhamnolipid solutions having a pH of 7 and a concentration of ca. 50% by weight are obtained. A disadvantage of this product form is the undesired, very sharp drop in viscosity on dilution. A further disadvantage is the reduced microbiological stability in the neutral state.